The present invention relates to a magnetic recording medium, having high electromagnetic transfer characteristics and durability and comprising one or more magnetic layers where ferromagnetic powder and a binder are dispersed, and said magnetic layers are placed on a non-magnetic support member.
The magnetic recording medium is widely used as recording tape, video tape or floppy disk. In the magnetic recording medium, a magnetic layer where ferromagnetic powder is dispersed in a binder is deposited on a non-magnetic support member.
The magnetic recording medium must be at high level in electromagnetic transfer characteristics, running durability and running performance. Specifically, in an audio tape for recording and reproducing musical sound, higher ability to reproduce the original sound is required. In a video tape, high ability to reproduce the original picture and excellent electromagnetic transfer characteristics are required.
In addition to the high electromagnetic transfer characteristics, the magnetic recording medium must have good running durability as described above. To provide good running durability, abrasive material and lubricant are added to the magnetic layer.
However, to have good running durability by adding an abrasive material, it is necessary to increase the adding quantity of the abrasive material to some extent, and this leads to the decrease of filling degree of the ferromagnetic powder. Also, in case the abrasive material having larger particle size is used to provide higher running durability, abrasive material may protrude excessively on the surface of the magnetic layer. Therefore, the improvement of running durability by the use of abrasive material causes problems such as deterioration of the electromagnetic transfer characteristics.
In case it is tried to improve the running durability by adding a lubricant, it is necessary to increase the adding quantity. This makes the binder more easily plasticizable and may decrease the durability of the magnetic layer.
In improving the durability and the electromagnetic transfer characteristics as described above, the binder, serving as one of major components of the magnetic layer, naturally plays an important role. Vinyl chloride resin, cellulose resin, urethane resin, acrylic resin, etc. as used in the past have caused problems such as deterioration of abrasion resistance of the magnetic layer or contamination of running members of the magnetic tape.
To solve the above problems, a method to use a harder binder and to increase hardness of the magnetic layer has been adopted, and it is proposed to produce a magnetic recording medium which uses a binder comprising a polyurethane resin with high hardness having an ether group-containing urethane of cyclic structure. For example, JP-A-1251416 describes a magnetic recording medium where a binder is used, which comprises a polyurethane resin containing polycarbonate structure by 5-50%. Also, JP-A-61148626 discloses a magnetic recording medium where a binder is used, which comprises a bisphenol type polyurethane resin containing polyol by 20 weight % or more. Further, JP-A-1267829 (U.S. Pat. No. 5,153,071) teaches a magnetic recording medium where a polyurethane is used, which contains a polyether polyol having cyclic structure.
The magnetic recording medium using these polyurethane resins as binder has high durability, while its dynamic strength at high temperature is not high. In particular, in an equipment used for high density recording, number of revolutions of magnetic head is high. The number of revolutions of the magnetic head is 9,600 rpm in a digital video tape recorder, 1,800 rpm in an analog video tape recorder for domestic use and 5,000 rpm for video tape recorder for business use. As a result, sliding speed between the magnetic recording medium and the magnetic head increases. Heat is generated due to sliding of the head where the tape is running at high speed, and magnetic coating film is fluidized, thereby leading to contamination of the head.
It has been proposed to increase heat resistance, moisture resistance, hydrolysis resistance, etc. by the use of a polyether polyester urethane resin. For example, JP-A-61104325 discloses a magnetic recording medium where a binder comprising a polyether polyester urethane resin is used, which contains a polyether polyol having an aromatic residual group. Also, JP-A-62121922 describes a magnetic recording medium where a binder comprising a urethane resin is used, which contains a polyether polyester polyol having aromatic hydrocarbon residual group. In the polyurethane described in JP-A-61104325, ethylene oxide, propylene oxide addition product, aromatic polybasic acid, etc. are used with PTMG (polytetramethylene glycol) and phenolic OH, and this has high heat resistance and hydrolysis resistance. Although there is no description on polyol content, it is about 55 weight % when calculated from the examples, and durability at high temperature is not sufficient. Because the content of ether group, serving as hydrophilic segment, is 7 mmol/g, it has low solubility in solvent, and dispersion property of the magnetic member is not satisfactory. The product disclosed by the latter patent has also similar problems.
JP-A-3201211 discloses a magnetic recording medium using a urethane resin, which contains a polyether block and a polyester block. PPG, PTMG, etc. are used as the polyether polyol, and PCL, butane diol adipate, etc. are used as polyester polyol, providing high hydrolysis resistance. However, the polyol content calculated from the examples is 60 weight % or more, and dynamic strength at high temperature is not sufficient.
In the magnetic layer using the polyurethane resins as given above, it is described that these are used alone or together with vinyl resin, but the disadvantage that the polyurethane resin has high strength but is fragile cannot be overcome.
To solve the above problems, it is an object of the present invention to provide a magnetic recording medium, which has high dispersion property and excellent electromagnetic transfer characteristics and also has coating film surface with high hardness at high temperature and superb durability at high temperature condition, and in particular, the invention provides a magnetic recording medium using a binder which contains a polyurethane having excellent properties.